Pres. Obama had a brief encounter today with Russian Pres. Vladimir Putin on the sidelines of a summit in China. White House officials say, the two leaders did not have time to discuss any of the long list of issues dividing the U.S. and Russia. Even in the area of nuclear security - once a high point in U.S.-Russian relations - things don't seem to be going well. And that worries nuclear experts in the U.S., as NPR's Michele Kelemen reports.

MICHELE KELEMEN, BYLINE: A former head of Los Alamos National Laboratories, Seigfried Hecker, is working on a book about the extraordinary cooperation between U.S. and Russian scientists in securing loose nuclear materials and upgrading former Soviet sites.

SEIGFRIED HECKER: Hopefully, the book will demonstrate to both governments it was absolutely essential for us to work together over these past 20 years, and that we're not done.

KELEMEN: Speaking from his office at Stanford, Hecker says that this cooperation has been tapering off in recent years and has now all but ended in the wake of the disputes over Ukraine and Crimea. He says, the U.S. is making it harder for Russian scientists to get visas to come here and vice versa.

HECKER: You know, there were times in the '90s when we'd have a hundred of our scientists and engineers at the various Russian facilities - the Russian institutes at various times. Today, if we have a few of them, you know, that's actually quite a bit.

KELEMEN: Hecker isn't alarmist about this, but says, it is important for nuclear scientists in both countries to learn new techniques on nuclear safety from each other.

HECKER: So I don't think anything terrible is going to happen overnight, but we're undermining safety and security of nuclear assets in Russia, for that matter, and in the United States.

KELEMEN: Russia's ambassador here in Washington, Sergey Kislyak, says, his country is committed to nuclear security and nonproliferation, but says, Russia is not planning to attend the Obama administration's next Nuclear Security Summit. He told reporters over lunch last week that Russia doesn't see much value in it and prefers to work through the International Atomic Energy Agency. The IAEA is a U.N. body, where Russia has more influence. Corey Hinderstein of the nonprofit group the Nuclear Threat Initiative says it's always better when world leaders come together around these issues.

COREY HINDERSTEIN: Is for the IAEA to remain effective, the leaders have to remain committed to its mission. And the summit has been a really important way of focusing that priority to the IAEA.

KELEMEN: So, she says, she's still hoping the U.S. will encourage Russia to take part.

HINDERSTEIN: The door's still open, and the administration has made clear that Russia will be invited to each of the subsequent planning meetings and to the summit itself. And I hope that sometime over the next two years, Russia will change its mind and decide that it does benefit - as do all of the participant states - in the work of the Nuclear Security Summit.

KELEMEN: But one of her colleagues at NTI, a former U.S. air attache to Moscow, Robert Berls, says it's been difficult talking to his Russian counterparts about these and other issues.

ROBERT BERLS: We seem to be doing more and more to disconnect our relationship, rather than connect our relationship.

KELEMEN: And that's risky, Berls says, at a time when both the U.S. and Russia have been investing heavily in their nuclear arsenals.

BERLS: You know, Russia is going through a major upgrade of its defense capabilities - building new missiles, new submarines, new heavy bombers, as well as revamping their defense industries.

KELEMEN: Berls says, he and other arms control experts thought they'd been moving in the right direction when Russia phased out one missile capable of carrying multiple warheads. But now, he says, the Russians are testing a new one that's even more capable. Michele Kelemen, NPR News, Washington. Transcript provided by NPR, Copyright NPR.